Reduction of weight of members such as cross members and side members of automobiles has been considered so as to support trends for reduction of fuel consumption in recent years. In terms of materials, from the viewpoint of securing strength and impact safety even when being thinned, a steel sheet has been made higher in strength. However, the formability of materials deteriorates along with the rise of strength thereof. In order to implement lighter weight of the members, a steel sheet which satisfies both of the press formability and the high strength has to be produced. In particular, excellent ductility is required in the case of forming the structural member or the reinforcing member of automobiles that has a complicated shape.
Recently, as a frame member of the automobile, a steel sheet having ultimate tensile strength of 440 MPa or 590 MPa is mainly used. In order to further reduce the weight, development of a high strength steel sheet of 980 MPa or more is preferred. In particular, when the steel sheet of 590 MPa class is replaced with the steel sheet of 980 MPa class, it requires an elongation equivalent to the 590 MPa class. Thus, development of a steel sheet of 980 MPa or more having excellent elongation is desired.
As the steel sheet having excellent total elongation (El) in a tensile test, there are complex structure steel sheets in which a soft ferrite serving as a primary phase is used in a steel sheet structure to ensure the ductility and a residual austenite is dispersed as a second phase to ensure strength.
As the steel obtained by dispersing the residual austenite, there is a TRIP (TRansformation Induced Plasticity) steel which uses martensite transformation of the residual austenite at the time of plastic processing, and applications thereof has been expanded, recently.
In particular, the TRIP steel has excellent elongation compared with precipitation strengthened steel and DP steel (steel consisting of ferrite and martensite), and thus expanded application is highly desirable. However, since this steel ensures excellent formability using martensite transformation at the time of forming, large amounts of residual austenite are required to ensure the formability. In order to ensure the residual austenite, it is required to add large amounts of Si. Further, in order to ensure the strength of 980 MPa or more, there is a tendency that alloy elements are added in large amounts and a texture easily develops. In particularly, the development of the texture of orientation groups {100} <011> to {223} <110> or the texture of an orientation {332} <113> promotes a material anisotropy. For example, as compared with the total elongation in the case of performing the tensile test on a steel strip in a direction parallel to a rolling direction, the total elongation in the case of performing the tensile test in a vertical direction is greatly inferior. Consequently, even though the steel sheet has characteristics of a good elongation in one direction and an excellent formability, it is difficult to apply to a member having a complicated shape.
On the other hand, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet having excellent corrosion resistance has been known as a steel sheet for automobile. The steel sheet is provided with a plated layer made of a zinc containing Fe of 15% or less on a surface of the steel sheet to have the excellent corrosion resistance. The hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet are manufactured in a continuous-type hot-dip galvanizing line (hereinafter, referred to as a CGL) in many cases. In the CGL, the steel sheet is degreased, then is annealed by an indirect heating with radiant tubes under a reducing atmosphere which contains H2 and N2, then is cooled to near a temperature of a galvanizing bath, and then is dipped in a hot-dip galvanizing bath. Thereafter, the steel sheet is subjected to a plating treatment.
In the case of manufacturing the alloyed hot-dip galvanized steel sheet, the steel sheet is dipped in the galvanizing bath and is then re-heated, so that the galvanized layer is subjected to an alloying treatment. At this time, an in-furnace atmosphere is an atmosphere at which Fe is reduced, and the steel sheet can be manufactured without oxidizing Fe, so that it is widely used as a manufacturing facility of a galvanized steel sheet.
However, Si is easily oxidized compared with Fe, and Si oxide is formed on the surface of the steel sheet while passing through the CGL. The Si oxide is responsible for galvanizing faults due to poor wettability with the hot-dip galvanizing. Alternatively, since the oxide inhibits an alloying reaction of Fe and zinc, there has a problem in that the alloyed hot-dip galvanized steel sheet cannot be manufactured.
With respect to this problem, a method of achieving both of the excellent formability and a plating property, in particular, a means of improving the plating property of steel containing Si in large amounts is disclosed in Patent Literature 1 in which annealing is performed once, then pickling is performed to remove the oxide on the surface of the steel sheet, and then the hot-dip galvanizing treatment is performed once again. However, this method is undesirable in that the annealing of two times is performed, and thus the pickling after heat treatment and a passage of the galvanizing line leads to a significant increase of processes and an increase of cost.
As the means of improving the plating property of the steel containing Si, a method of suppressing oxides of Si or Mn by making in-furnace atmosphere to be a reducing atmosphere of easily oxidizable elements such as Si and Mn or a method of reducing the formed oxides is disclosed in Patent Literature 2. In this method, pre-plating or surface grinding is performed on materials having a poor plating property prior to entering the galvanizing line. However, as a process of the pre-plating or the surface grinding increases, the cost increases. In addition, since the high strength steel sheet generally contains Si and Mn in large amounts, it is difficult to achieve an atmosphere capable of reducing Si in the steel sheet containing Si of 0.5 wt % or more which is a target of the present invention, and thus huge facility investment is required, resulting in increasing the cost. In addition, since oxygen released from the reduced oxides of Si and Mn changes the in-furnace atmosphere into an oxidizing atmosphere of Si, it is difficult to stabilize the atmosphere in the case of performing massive production. As a result, there is a concern in that defects such as unevenness of plating wettability or alloying unevenness occur in a longitudinal direction or a width direction of the steel sheet.
As a means of achieving both of the excellent ductility and a plating property, Patent Literature 3 discloses a method in which cold-rolling is performed, then the surface of the steel sheet is subjected to a pre-plating treatment with metals such as Ni, Fe, or Co and is subjected to a plating treatment while passing through a heat-treatment line. This relates to a method of pre-plating metals, which are difficult to oxidize compared with Si and Mn, on a surface layer of the steel sheet and manufacturing the steel sheet not containing Si and Mn on the surface layer of the steel sheet. However, even when the surface of the steel sheet is subjected to the pre-plating treatment, these elements diffuse into the inside of the steel sheet during the heat treatment, and thus a large amount of pre-plating should be performed. Therefore, there is a problem in that the cost remarkably increases.
As a means of solving these problems, Patent Literatures 4 to 6 propose a method in which Si oxide is not formed on the surface of the steel sheet but is formed inside the steel sheet. This can increase oxygen potential in the furnace and can oxidize Si inside the steel sheet to suppress diffusion of Si into the surface of the steel sheet and formation of the Si oxide on the surface.
In addition, Patent Literatures 7 and 8 do not relate to TRIP steel but to galvanized steel sheet and disclose a method of setting the inside of the furnace to be the reducing atmosphere at an annealing process of CGL. Moreover, Patent Literature 9 discloses a method of providing a jet flow of a predetermined flow rate in a galvanizing bath to prevent galvanizing faults by scum.
However, the conventional techniques are extremely difficult to simultaneously provide the corrosion resistance, the high strength, and the ductility.